1. Field of the Invention
The present invention relates to a method of conditioning an RF-powered plasma processing chamber after cleaning the interior chamber walls.
2. Prior Art
Many thin film processes use plasma processes to facilitate the rapid and accurate fabrication of minute structures with desired properties. Plasma processes include the deposition and etching of insulators, conductors and semiconductors on a substrate, for example, a silicon wafer. The plasma process usually involves placing the substrate in a vacuum chamber, introducing process gases and applying radio-frequency (RF) power, typically 0.1 to 200 MHz, to create a plasma. The plasma consists of ions, electrons, radical gas species and neutral gas, all of which permit the desired reaction to proceed. The plasma reaction has many inputs, including RF power, gas type and flow rates, chamber pressure, substrate and wall temperatures, chamber wall conditions, electrode spacing, and so on.
The chamber configuration and chemistry used is chosen according to the desired process. For example, plasmas are used to etch dielectrics in semiconductor manufacture using specific plasma chamber designs such as Reactive Ion Etching (RIE) or Inductively Coupled Plasma (ICP) and using etching gases such as CHF3, CF4, O2 and so on.
During the manufacturing process, by-products of the reaction process, for example polymers, are deposited on the walls of the chamber. Typically, a wet clean is performed to remove these by-products. If this material were not periodically removed, it would eventually flake, depositing particles onto the wafer surface, and impacting yield.
When the reactor walls have been cleaned, and vacuum integrity has been restored, the chamber undergoes a conditioning process, during which non-product (conditioning) wafers are cycled through the chamber according to a predetermined conditioning recipe. As is well-known in the art, a conditioning wafer is a non-product wafer with a substrate which is similar to that used in production on the same chamber, for example polysilicon, silicon dioxide, silicon nitride or bare silicon. A conditioning recipe is a recipe similar or identical to that used in production on the same chamber, and is also referred to as a burn-in recipe. The objective of conditioning is to deposit an initial layer of polymer material on the clean chamber walls. If the chamber is not conditioned in this way, the first number of production wafers experience significantly different process conditions, resulting in yield loss.
After the conditioning cycle, a test or qualification wafer may be run through the chamber to confirm that process conditions have reached equilibrium. If not, the chamber is conditioned further. The operator of the chamber does not know exactly how many conditioning wafers are needed to restore process conditions. Operating procedures usually call for a fixed number of conditioning wafers to be run on all chambers of the same type. For example, it may be that twenty-five wafers each running the conditioning recipe for one minute is advised. This is not an optimal method, since equilibrium conditions are often unpredictable, being a function of wafer grade, chamber configuration or the time the chamber has been exposed to atmosphere.
Preventive maintenance is classified as downtime, during which no production wafers can be processed in the chamber. The conditioning step of the preventive maintenance activity can constitute a large percentage of the total preventive-maintenance downtime, thus incurring a significant cost. There is a need therefore, for some method of determining when a chamber is conditioned, and ideally advising operators in advance.
It is known that certain electrical signals derived from the plasma power source can be sensitive to many plasma processing events. The plasma represents a non-linear complex load in electrical terms. This results in the generation of harmonics of the RF driving signal. These harmonics, known as Fourier components, are very sensitive to changes both in the plasma process and the process parameters. It is generally accepted that monitoring the Fourier components of the RF power signal provides a useful way to monitor the plasma process. These components are a more direct measurement of the plasma process since they are more directly related to fundamental plasma parameters.
It is known to use an RF sensor to monitor and control RF plasmas by measuring the Fourier components of voltage and current. The sensor can be used in closed or open loop control, as for example, in etch end-point control or as in-situ monitoring of the plasma process. In either case the plasma can be terminated when one or more of the RF Fourier components reaches pre-determined limits.
In particular, it has been suggested (xe2x80x9cMeasurement of discharge impedance for dry etch process controlxe2x80x9d, F. Bose et al., SPIE Proceedings, vol 2336, pp101-110, 1994) that chamber conditioning can be stopped when the RF parameters reach a pre-determined stable value. The concept is that wall conditions have stabilised. One limitation of this approach is that the operator of the chamber has no advance warning of when this event may occur, and cannot decide to continue conditioning the chamber, load qualification wafers or load production wafers into the tool. It would be much more useful for the operators of plasma-processing chambers to have a system that predicted the number of conditioning wafers needed.
Accordingly, the present invention provides a method of conditioning an RF-powered plasma processing chamber comprising the following steps:
(a) after cleaning the interior chamber walls, running a pre-determined number of conditioning wafers successively through the chamber each using substantially the same conditioning recipe;
(b) in respect of each such conditioning run, determining the magnitude of a Fourier component of the delivered RF power;
(c) based on the magnitudes determined in step (b), building a predictive model of Fourier component magnitude as a function of the number of conditioning wafers run; and
(d) determining the number of conditioning wafers needed to be run for the rate of change of the predicted magnitude of the Fourier component to reach a pre-determined threshold level.
This invention provides several advantages over the existing known methods. Firstly, the number of conditioning wafers may be optimised so that the amount of downtime is minimised. Secondly, the number of qualification wafers is reduced as these wafers are loaded only when the predictive model indicates that the chamber will reach equilibrium.